In recent years, highly efficient Ultra Super Critical Boilers, with advanced steam temperature and pressure, have been built in the world. Specifically, it has been planned to increase steam temperature, which was about 600° C., to 650° C. or higher or further to 700° C. or higher. Energy saving, efficient use of resources and the reduction in the CO2 gas emission for environmental protection are the objectives for solving energy problems, which are based on important industrial policies. And further, a highly efficient Ultra Super Critical Boiler and a reactor are advantageous for a power generating boiler and a reactor for the chemical industry, which burn fossil fuel.
High temperature and high pressure steam increases the temperature of a superheater tube for a boiler and a reactor tube for the chemical industry, and also high temperature machines and equipment constructed from thick plates, forgings and so on, which are used as heat resistant pressurized members, during a practical operation to 700° C. or higher. Therefore, not only the high temperature strength and high temperature corrosion resistance, but also the excellent stability of a microstructure for a long period of time and creep properties are required for the alloy used in such a severe environment.
Thus, the Patent Documents 1 to 3 disclose heat resistant alloys in which the contents of Cr and Ni are increased. Moreover, they additionally contain one or more of Mo and W, in order to improve the creep rupture strength which is a sort of high temperature strength.
Furthermore, from the viewpoint of increasingly demanding requirements for high temperature strength characteristics, especially the requirement for creep rupture strength, the Patent Documents 4 to 7 disclose heat resistant alloys which contain 28 to 38% of Cr and 35 to 60% of Ni by mass percent and exploit the precipitation of the α-Cr phase, which has a body-centered cubic structure and comprises mainly Cr, in order to ensure further improvement in creep rupture strength.
On the other hand, the Patent Documents 8 to 11 disclose Ni base alloys which are used in the above-described severe high temperature environment. These alloys contain Mo and/or W in order to achieve a solid solution strengthening effect, and contain Al and Ti in order to utilize the precipitation strengthening effect of the y′ phase, which is an intermetallic compound, specifically, Ni3(Al, Ti).
Also, the Patent Document 12 proposes a high-Ni austenitic heat resistant alloy in which the addition range of Al and Ti is regulated and the Y′ phase is precipitated in order to improve the creep strength.
Patent Document 1: JP 60-100640 A
Patent Document 2: JP 64-55352 A
Patent Document 3: JP 2-200756 A
Patent Document 4: JP 7-216511 A
Patent Document 5: JP 7-331390 A
Patent Document 6: JP 8-127848 A
Patent Document 7: JP 8-218140 A
Patent Document 8: JP 51-84726 A
Patent Document 9: JP 51-84727 A
Patent Document 10: JP 7-150277 A
Patent Document 11: JP 2002-518599 A
Patent Document 12: JP 9-157779 A